The present invention is directed to an FMCW sensor.
Given sensor systems with FMCW radar, particularly microwave radar systems, it is possible to obtain simultaneous range and velocity measurements. Such sensor systems have versatile use possibilities, for example in the automotive or automation technologies. The abbreviation FM stands for frequency modulation and CW stands for continuous wave. A signal source generates a frequency-modulated signal that propagates wave-like. The signal source comprises, for example, a microwave oscillator and a modulator. The preferably linearly frequency-modulated signal is beamed out by an antenna and is reflected from a target. The received signal is mixed in a mixer with the transmission signal present at the moment. The difference frequency that derives is a measure for the distance of the target from the antenna. A monostatic or a bistatic antenna arrangement can be employed for the transmission and reception means. Given the monostatic arrangement, the transmission and the reception signals are beamed out or, received by the shared transmission and reception antenna. As a result of a circulator or directional coupler, the transmission and the reception signals are separated from one another, i.e. the transmission signal is separated by the reception mixer and the reception signal is separated from the transmission path. Given the bistatic antenna arrangement, separate transmission and reception antennas are provided.
PCT Patent Application No. 97/09637 discloses an FMCW radar sensor wherein a delay line is present as a reference means. A time-delayed signal is generated from the transmission signal with this reference means, a balancing and a correction of a measured signal being capable of being undertaken with said time-delayed signal such that a non-linear curve of the characteristic of the modulation can thus be computer-compensated. A correction of the modulation characteristic on the basis of an evaluation of the reference signal is likewise possible. PCT Patent Application No. 97/09777 discloses a device serving a similar purpose.
Further examples for linearization of the modulation given FMCW sensors may be found in the publications by B. Zimmermann et al., "24 GHz Microwave Close-Range Sensors For Industrial Measurement Applications" in Microwave Journal, May 1996, pages 228 through 238, and by P. Lowbridge et al., "A Low Cost mm-Wave Cruise Control System for Automotive Applications" in Microwave Journal, October 1993, pages 24 through 36. A closed control loop, for example with digital frequency discriminator, is employed, this being supplied with an intermediate frequency signal. An additional, frequency-stable local oscillator is required for this purpose. As an alternative thereto, U.S. Pat. No. 5,546,088 discloses a radar sensor wherein a frequency-modulated 24 GHz radar signal is generated by five-fold frequency multiplication of a 4.8 GHz VCO signal. However, a considerable circuit outlay is required for this purpose.
What is also disadvantageous about known FMCW sensors is that separate transmission and reception devices must be present given the bistatic structure and a circulator or hybrid coupler or, directional coupler for dividing transmission and reception signals onto the various signal paths is required given the monostatic structure. German Patent Application No. 196 10 850 discloses a monostatic, homodyne radar system, whereby a balanced mixer (push-pull mixer) is utilized instead of a traditional directional coupler with separate mixer. The mixer contains a hybrid coupler functioning as directional coupler with 90.degree. phase difference between coupling arms, and comprises a reflection factor of more than 0.3, so that a coupling of the power supplied by the oscillator to the antenna ensues that is adequate for the emission of the transmission signal.
PCT Patent Application No. 95/26073 discloses a radio-frequency oscillator in planar structure, whereby a dielectric resonator is operated in higher modes and is coupled by suitable alignment to a micro strip conductor. Such an oscillator in the region around 24 GHz is suitable as a signal source for FMCW sensors.